1. Field of the Invention
The present invention relates to an image forming apparatus and a developing device including a rotator and a bearing element that supports a rotating shaft of the rotator.
2. Description of the Related Art
Ball bearings and sliding bearings are used in image forming apparatuses at various locations. For example, ball bearings are used at a location in which a rotating shaft is supported. On the other hand, sliding bearings in which a rotating shaft rotates while sliding a sliding surface of the sliding bearing. Ball bearings are complicated in structure and therefore expensive than sliding bearings. Therefore, use of ball bearings is generally restricted to special situations i.e., where a comparatively large load exists. Bearing structures using the sliding bearing are described in FIGS. 8 and 9 (not shown) in Japanese Patent Application Laid-open No. 2001-125374.
The bearing structure of FIG. 8 includes sliding bearings provided at both ends of a rotator in the shaft direction, and this structure can reduce the cost as compared with the bearing structure using the ball bearing. The bearing structure of FIG. 9 includes a ball bearing provided at one end of a rotator in the shaft direction and a sliding bearing at the other end thereof. If drive is transferred to the rotating shaft from the outer sides of the bearings in the shaft direction of the rotator, a portion of the rotating shaft supported by the bearing on a drive input side is applied with a load larger than that of a portion thereof supported by the other bearing. In this case, as described in FIG. 9, by using the ball bearing for the bearing on the drive input side, durability of the bearing on the drive input side is improved more than that of the case in which the sliding bearings are used for both the two bearings, and this allows improvement of the durability of the entire bearing structure. Besides, cost reduction can be achieved as compared with the case in which the both bearings are provided with the ball bearings.
As described in FIGS. 8 and 9, by providing the sliding bearing for the both of the bearings or by providing the sliding bearing for one of the bearings depending on the intended use, the cost reduction of the bearing structure can be achieved without impairing durability of the bearing structure, as compared with the structure in which both of the bearings are provided with the ball bearings.
However, by continuously using a developing device that includes the bearing structure described in FIG. 8 or FIG. 9 over long periods, trouble such as abnormal noise has occurred from the bearing portion on the sliding bearing side.
The trouble due to its long-term use may occur not only in the bearing portion of the developing device, but also in any bearing portion that supports the rotating shaft of a rotator to a housing of the device.
The situation in which the abnormal noise has occurred indicates a situation in which frictional force between the rotating shaft and the bearing element increases to such an extent that the abnormal noise will occur. Therefore, if the situation is left as it is, the rotating shaft is locked to the bearings, which may lead to fatal damage of a drive source of the rotator or the like.